Phosphate binding to allophane and ferrihydrite with implications for volcanic ash soils

Abstract

AbstractThe occurrence of allophane and ferrihydrite in volcanic ash soils contributes to the high P sorption capacity of the soils. The objective of this study was to investigate the distribution of phosphate between allophane and ferrihydrite in the systems of their mixtures as a function of sorbed phosphate and aqueous citrate concentrations. The results were compared with those from phosphate in volcanic ash soils rich in allophane and ferrihydrite. Phosphate adsorption isotherm for allophane–ferrihydrite mixtures was described as a linear combination of Freundlich isotherm models for each single compound system. Phosphate sorption across single‐ and binary‐compound systems decreased with increasing aqueous citrate concentrations. Phosphorus K‐edge X‐ray absorption near edge structure (XANES) showed that phosphate had a higher affinity for allophane than ferrihydrite, particularly at low to intermediate sorbed phosphate concentrations. Such a preferential distribution of phosphate for allophane were attenuated at 10 mM aqueous citrate with increasing sorbed phosphate concentrations. Changes in the full width at half‐maximum height of the white‐line peak in the P K‐edge XANES spectra provided evidence for Al‐phosphate precipitation in the systems of allophane and allophane‐ferrihydrite mixtures, but not in the ferrihydrite systems. The XANES results from volcanic ash soils demonstrated that 61–92% phosphate was associated with Al, thus confirming a high affinity of phosphate for Al over Fe in the soils.